In a recently completed study, we found that in rats that received programmed infusions of 1 mg/kg cocaine every 12 min for 2 hr over 10 days, the pre-infusion concentrations of prolactin (PRL) increased in a time-dependent manner whereas post-infusion levels of PRL were decreased by cocaine. Because dopamine (DA) and PRL are reciprocally related in male rats, these changes could involve modification of adenohypophysial dopamine D2 receptors. Dispersed anterior pituitary cells were obtained from rats treated chronically with cocaine or saline. The cells were used in a reverse hemolytic plaque assay that permitted direct visualization of PRL release from single lactotropes after different treatments in vitro. The cells were incubated with media, cocaine, thyrotropin releasing hormone (TRH), or dopamine (DA) in vitro. There were 4 major findings. 1) Basal PRL release was greater in rats treated with cocaine: more cells secreted PRL and the individual cells secreted more PRL. 2) Cocaine in vitro did not affect PRL release. 3) TRH stimulated PRL release similarly from lactotropes of cocaine- or saline-treated rats. 4) DA in vitro inhibited PRL release dose-dependently from both cocaine- and saline-treated rats when the concentration of DA met or exceeded that observed in hpothalamo-hypophysial portal blood. However, lactotropes from cocaine-treated rats were more sensitive to the inhibition by DA. Paradoxically, very low concentrations of DA (<10-9M) enhanced PRL release from cells from cocaine-treated rats. These data confirm the findings of others that DA-deprived lactotropes release more PRL when challenged with low concentrations of DA and suggest that one consequence of chronic use of cocaine is a diminished release of DA in the absence of cocaine.